Electrodes for use in alkali storage cells are classified into a sintered type and a paste type. The sintered-type electrodes have excellent current collecting performance, but they require a complicated operation for being filled with active material and cannot easily achieve high energy density. In contrast, the paste-type electrodes have good workability, allowing the substrates to be directly filled with active material and achieve dense filling of active material.
As substrates for use in the paste-type electrodes, porous metal plates and spongelike metal substrates have been used. The term "spongelike" refers to a state where a multiplicity of cells are contiguous to each other and fluid such as air or paste inside these cells can travel from one cell to another. Spongelike metal substrates made of nickel or the like (foamed nickel substrates or the like) have become the mainstream, with the growing demands for cells which have higher energy density and can be manufactured at a lower cost. The reason for the popularity of the spongelike metal substrates is due to their favorable features as follows. The spongelike metal substrates contain numerous three-dimensional spaces each of which is surrounded by metallic lattices and can be filled with solid active material in paste form directly and at high density. Furthermore, contiguous numerous metallic lattices serve as a current collector, making it unnecessary to use a conductive supporter.
Generally, a spongelike nickel substrate is prepared as follows: A spongelike organic high polymer (organic polymer, organic macromolecular substance) such as foamed urethane is plated with nickel by either electroplating, electrolessplating, or vapor deposition, then the nickel-plated high polymer is baked at a high temperature. The electrode substrate thus produced is highly porous and contains numerous metallic lattices which are composed of thin metal wires. This electrode substrate can be filled with active material at high density; however, its stretching force is weak because the lattices are composed of extremely thin metal wires. As a result, the lattices are susceptible to fracture in parts during electrode pressing process or electrode coiling process after the electrode substrate has been filled with active material. The fracture of the metallic lattices leads to the deterioration of the current collecting efficiency, and as a result, there is a problem that the electrode performance cannot improve sufficiently even if the electrode substrate is filled with active material at high density.
There is another problem that cells which employ such a spongelike nickel substrate do not necessarily achieve an expected high-temperature consecutive charge characteristic, whose cause has been unknown.